October 13 to November 11: Longest lunar month of 2015

Longest lunar month of 2015 starts with the October 13 new moon and ends with the November 11 new moon. Learn about the varying lengths of lunar months, here.

Simulated view of the moon’s phases. The period of time from new moon to new moon is known as the lunar month, lunation or synodic month. From the years 1760 to 2200, the longest lunar month spans 29 days 19 hours and 58 minutes (Dec. 9, 1787 to Jan. 8, 1778) while the shortest lasts for 29 days 6 hours and 34 minutes (June 12 to July 12, 1885).

The longest lunar month of 2015 begins with the October 13 new moon at 00:06 UTC (October 12 at 7:06 p.m. CDT). It ends with the new moon of November 11. Follow the links below to learn more:

One lunar month is the period of time from new moon to new moon. As viewed from the north side of The Earth’s and moon’s orbital planes, the Earth goes counterclockwise around the sun and the moon goes counterclockwise around Earth. Image via Wikipedia

Shortest lunar month of 2015. The shortest lunar month this year happens in between the new moons of April 18 and May 18. This lunar month is only 29 days 9 hours and 16 minutes long, or 3 hours and 28 minutes shorter than the mean lunar month of 29 days 12 hours and 44 minutes.

What is a lunar month? It’s just the duration between successive new moons. It’s also sometimes called a lunation or synodic month. Although the lunar month has a mean period of 29.53059 days (29 days 12 hours and 44 minutes), the actual length varies throughout the year.

Longest lunar month of 2015. The longest lunar month of 2015 – October 13 through November 11 – lasts for 29 days 17 hours and 41 minutes.

That’s 4 hours and 57 minutes longer than the mean lunar month of 29 days 12 hours and 44 minutes.

All added up, that means the year’s longest lunar month in 2015 exceeds the year’s shortest by 8 hours and 25 minutes.

The moon’s orbit around Earth is not a perfect circle. But it is very nearly circular, as the above diagram shows. Diagram by Brian Koberlein.

Why the difference in the lengths of lunar months? In a nutshell, the longest lunar month of the year occurs when the successive new moons coincide closely with lunar apogee – the moon’s farthest point to Earth in its orbit.

In contrast, the year’s shortest lunar month takes place when the successive new moons fall appreciably close to lunar perigee – the moon’s closest point to Earth in its orbit.

On the average, the lunar month (new moon to new moon) is about 2.22 days longer than the sidereal month (one complete revolution of the moon relative to the background stars). However, if the moon is near apogee at the end of one sidereal month, the moon travels more slowly than average in its orbit. Therefore, the period of time between the end of the sidereal month and the end of the lunar month is longer than average.

The opposite is the case when the moon is near perigee. The moon travels more swiftly in its orbit at perigee, in which case the time period between the end of the sidereal month and the end of the lunar month is less than average.

In 2015, the year’s longest and shortest lunar months do not actually showcase great extremes. In fact, the longest and shortest lunar months in the years ahead will vary more greatly from the mean.

The longest lunar months happen when successive new moons occur near lunar apogee – and in addition, the Earth is near perihelion (Earth’s closest point to the sun in its orbit). Because Earth is always closest to the sun in early January, the very longest lunar months take place in between December and January new moons.

On the other hand, extremely short lunar months happen when successive new moons fall near lunar perigee – and in addition, the Earth is near aphelion (Earth’s farthest point from the sun in its orbit). Because Earth is always at aphelion in early July, the very shortest lunar months take place in between June and July new moons.

When are the longest and shortest lunar months of 21st century? The longest lunar month of the 21st century (2001 to 2100) occurs in between the December 2017 and January 2018 new moons. With a length of 29 days 19 hours and 47 minutes, this particular lunar month exceeds the mean by a whopping 7 hours and 3 minutes.

The century’s shortest lunar month takes place in between the new moons of June and July 2053, a period of 29 days 6 hours and 35 minutes. That’s 6 hours and 9 minutes shorter than the mean.

Incidentally, exceptionally long or short lunar months repeat in cycles of 9 years.

Each year, the shortest and longest lunar months come later in the year. For instance, in 2016, the shortest lunar month happens in between the May 6 and June 5 new moons; and the longest one in between the October 30 and November 29 new moons. Click here for a complete listing for the length of each lunar month in the 21st century.

Bottom line: A lunar month is the duration between successive new moons. It’s also sometimes called a lunation or synodic month. Although the lunar month has a mean period of 29.53059 days (29 days 12 hours and 44 minutes), the actual length varies throughout the year. The shortest lunar month of the year begins with the new moon of April 18, 2015 and ends with the new moon of May 18, 2015.

Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.